In active depth sensing, such as used by active stereo systems, a projector projects patterns of light such as infrared (IR) dots or lines to illuminate a scene being sensed. Note that the projector may use a laser that consumes on the order of 1 W of power, which means that the projector consumes too much power to be practically built into in a small satellite device such as a smartphone or tablet, and instead is typically part of a device plugged into a wall socket.
The projected patterns are then captured by a camera/sensor (two or more in stereo systems), with the image (or images) processed to compute a depth map or the like. For example, in stereo systems, stereo cameras capture two images from different viewpoints. Then, for example, one way to perform depth estimation with a stereo pair of images is to find correspondences between the images, e.g., to correlate projected and sensed dots in the left image with counterpart dots in the right image. Once matched, the projected patterns within the images may be correlated with one another, and triangulation, possibly along with disparities between one or more features of the correlated dots (e.g., including their intensities) may be used to estimate a depth to an object struck by that particular projected dot.
In most scenarios, the cameras that capture the stereo images are arranged to sense a relatively large space, such as a room, which can be relatively large. As a result, an object in the distance, such as a face, appears in only a relatively small number of camera pixels. Sufficient detail and/or accurate depth estimation is thus not available as needed by many applications, such as face recognition and so forth.